This invention relates in general to surface mount and plated through hole technology printed wiring boards, and in particular to a printed wiring board structure interfaced with an integral core fabricated of a metal matrix with pitch based graphite fibers therein disposed, with the board structure and core integrally connected to each other at a plurality of connection sites along the interface thereof.
Printed wiring board structures generally are constructed of a core upon which one or more layers carrying ceramic chips or leaded components are laminated. These circuit and chip-carrying layers usually are of metalized (copper clad) polymeric construction such as a polyamide, and can be stacked above and below the core material. In order to provide efficient operation within the wiring board structure, the core should perform favorably with respect to tensile modulus, thermal conductivity, and thermal expansion.
A common core construction now employed is a metal core fabricated as a layer of molybdenum having on each surface thereof a respective layer of copper. While this copper-molybdenum-copper core is satisfactory with respect to tensile modulus, thermal conductivity, and thermal expansion considerations, the weight of this prior-art core can be a significant disadvantage in weight-sensitive applications. Conversely, of course, any replacement core material whose attributes include light weight must still provide satisfactory strength, low coefficient of thermal expansion and heat-response characteristics in order to qualify for wiring board construction.
In view of the above described requirements, it is apparent that a need is present for a printed wiring board structure having a core fabricated to meet weight and expansion restraints while providing efficient wiring board performance. Accordingly, a primary object of the present invention is to provide an integrated printed wiring board structure with a core exhibiting high tensile modulus, high thermal conductivity, low coefficient of thermal expansion, and light weight while being compatible with both surface mount and plated through hole technology components and corresponding circuitry.
Another object of the present invention is to provide such a printed wiring board structure whose core construction includes a metal matrix having disposed therein pitch based graphite fibers.
These and other objects of the present invention will become apparent throughout the description thereof which now follows.
The present invention is a printed wiring board structure comprising at least one chip-carrying layer adjacent a core fabricated of a metal matrix having disposed therein graphite fibers. The chip-carrying layer and the core have an interface therebetween and are integrally connected to each other through vias plated with a thermally and electrically conductive material to thereby provide a plurality of connection sites along this interface. A metal matrix is preferably fabricated of aluminum. Preferred fibers are fabricated of pitch based graphite. A typically preferred present printed wiring board structure has two chip-carrying layers each on opposite sides of the core, with each of the layers and the core having respective interfaces therebetween wherein each layer is integrally connected to the core at a plurality of connection sites along the respective interfaces. Additional circuit layers can be affixed to those layers, allowing layer interconnection, and/or interface with the core, thereby increasing circuit density of a wiring board structure. The present printed wiring board structure possesses high tensile modulus, low coefficients of thermal expansion, and light weight to thereby provide versatility in printed wiring board structural utility and placement. Because of the plurality of connection sites along respective interfaces of the core and adjacent circuit and chip-carrying layers, superior thermal conductivity occurs from the layers to the core since heat travels through these connection sites to effectuate extremely efficient heat transfer and ultimate heat dissipation from the entire printed wiring board structure. In this manner reduced operating temperatures are accomplished to thereby yield higher reliability, improved dynamic load, improved thermal fatigue life of solder joints, and accommodation of more circuitry per unit area. In addition, weight savings of 30% to 40% can be realized over current printed wiring board structures.